1. Field of the Invention
This invention relates to tension mask color cathode ray tubes (CRTs) having a front assembly including a faceplate with a striped screen and a strip-type shadow mask mounted on mask support structures attached to the faceplate. The invention is addressed specifically to an improved means for damping the vibration of the strips of the mask.
2. Discussion of Related Art
A CRT and its Front assembly known in the art, and which utilizes a tensed strip shadow mask, is depicted in FIGS. 1 and 2. Cathode ray tube 20 has a front assembly 22 that includes a rectangular glass faceplate 24 which is sealed to a funnel 26. The x-axis of the Faceplate 24, commonly known as the horizontal axis, and the y-axis, commonly known as the vertical axis, are indicated in FIG. 2.
The neck 28 that extends from funnel 26 encloses an in-line electron gun 30 that projects three discrete electron beams 32 that excite the phosphor stripes deposited on screen 38 to produce a color image visible from the outer surface 40 of faceplate 24. A magnetic shield 42 internal to the funnel 26 and having walls approximately conforming to the shape of the funnel, provides for shielding the tube 20 from the effects of stray magnetic fields that may emanate from nearby electrical components, and from the influence of the terrestrial magnetic field. The centrally disposed striped screen 38 is deposited on a rectangular area on the inner surface 44 of faceplate 24. Striped screen 38 consists of patterns of spaced deposits of phosphor oriented in a first direction, indicated by FIG. 2 as being approximately along the y-axis, by way of example. Two shadow mask supports 48 and 50, hereafter termed "rails," are affixed to the inner surface 44 of faceplate 24 on opposed sides of the screen 38 in a second direction perpendicular to the first direction for receiving a shadow mask 52; the second direction is indicated in FIG. 2 as being along the x-axis. The mask 52 is in -turn affixed to the rails 48 and 50 preferably by welding the borders 54 and 56 of the mask 52 to the respective underlying rails 48 and 50 by means of a laser.
An inset 58 depicts a representative section of the shadow mask 52 greatly enlarged. Shadow mask 52 is shown as being a "strip mask" that consists of a plurality of strips 60 aligned in the first direction, and spaced apart as indicated by the intervening slits 62.
The strip mask 52 is formed from a metallic foil which may have a thickness in the range of 0.0003 inch to 0.005 inch, with the thickness dependent upon the size and application of the CRT. Such thin foils are basically non-self-supporting so they must be installed in a highly tensed state on the rails. Also, the magnitude of the tension must be high enough so that the tension is not lost when the mask expands thermally during operation. By way off example, and depending upon the thickness of the foil, the tension of a foil mask For a 14-inch (diagonal measure) CRT is about forty lb./in.
A form of the strip mask, the subject of U.S. Pat. No. 3,638,063 to Tachikawa et al, is shown by FIG. 3. The mask 66 consists of a parallel array of narrow strips 67, the ends of which are attached to curved supports 68 and 69. Tension is applied to the strips 67 of mask 66 by an outward pressure on supports 68 and 69 exerted by two U-shaped spring members 70 and 71. As a result, the mask is formed into into a sector of a cylindrical surface that conforms to the curvature of the associated faceplate (not shown). The disadvantages in a mask assembly of this type include its its bulk and weight, and the tendency of the strips 67 to vibrate and distort the image that is visible through the external surface of the associated curved faceplate.
The vibration problem is remedied in Tachikawa et al by the stretching of one or more fine wires, represented by the single wire 72, over the cylindrical surface of the mask, which serves to dampen vibration by contact with the strips 67. The wire 72 is shown as being are suspended between two spring members 74 and 75 attached to respective U-shaped spring members 70 and 71.
The problem of strip vibration is inherent in all types of strip mask, including the strip masks typified by those described and claimed in commonly owned U.S. Pat. Nos. 4,942,332 and 5,085,606. Damping vibration in strip masks of the type described in the patents presents problems due to the basic differences between the CRT described heretofore and Tachikawa et al. For example, the mask of the present invention is flat rather than curved, and the mounting means comprise rails that project from a flat faceplate. Further, there are no available structures nearby (as in Tachikawa et al) for the suspension of the wire. As a result, an inventive solution is required to achieve damping of strip vibration in flat strip masks mounted on rails secured to a flat faceplate.
This invention is applicable as well to a tension mask CRT having a slightly curved shadow mask, as disclosed in commonly owned referent copending application Ser. No. 07/974,443.